Overview
The indicating system gives the flight crew the data to monitor the conditions of the engine fuel system.
The indicating system gives the flight crew the data to monitor the rate of fuel flow in pounds per hour (pph) or kilograms per hour (kph). This data is found on the engine indicating system area of the engine indicating and crew alerting system (EICAS). The EICAS message display shows the related messages for the engine fuel indicating system.
The fuel indication system supplies data for the fuel flow of the engine, the fuel pressure to the inlet of the engine, and the fuel temperature from the fuel heater/ oil cooler (FHOC). This data is sent to the aircraft flight compartment and to the ECU. The fuel indication system also shows when the fuel flow decreases to less than a specified quantity and the condition of the fuel filter.
Fuel Pressure Switch
The fuel pressure switch is attached to the inlet of the hydromechanical unit (HMU) and connects to the fuel inlet line. The switch changes condition based on the fuel feed pressure and shows if there is low pressure at the inlet of the HMU. The fuel pressure switch closes at 8.0 psi (55.16 kPa) and opens at 6.0 psi (41.37 kPa). When the switch is closed, the switch is continuously monitored by a built-in test (BIT).
Fuel Flow Meter
The fuel flow meter is an impeller-type transmitter attached to the front outer fan duct. The fuel flow meter monitors the fuel flow from the HMU to the engine. The fuel flow meter gives electrical signals that are directly related to the mass of the fuel flow. The electrical signal is sent through the ECU to the aircraft flight compartment.
01/27/16
Operation
The fuel pressure switch supplies an electrical signal to the ECU of the fuel pressure which is supplied to the HMU. When the fuel pressure switch senses that the fuel feed pressure is less than 6.0 psi (41.37 kPa), the ECU sends a signal to show a L (R) FUEL LOW PRESSURE caution message on the EICAS. The caution message goes off again when the fuel pressure increases to more than 8.0 psi (55.16 kPa). When the L PUMP and/or R PUMP rotary switches on the fuel control panel are set to AUTO, the fuel feed system operates in automatic mode. In automatic mode, if an ejector pump fails, low pressure in the feed line causes the related dc boost pump to start automatically.
The fuel indication system gives the flight crew the data to monitor the rate of fuel flow in pounds per hour (PPH) or kilograms per hour (KPH). Fuel flow through the fuel flow meter generates electrical signals directly proportional to the mass of the fuel flow. The electrical signal is sent to the ECU where the signal is calculated and then transmitted to the EICAS displays. The fuel flow indication is shown in magenta if the fuel flow data or the units of measure is not correct. The fuel flow indication is green in all other conditions.
The dual function switch has a mechanical bypass function and a temperature measurement function. The mechanical bypass function occurs when the filter differential pressure gets to 14.0 ±1.5 psid (96.53 ±10.34 kPa). This will give an indication of an impending bypass of unfiltered fuel from the inlet to the outlet port because of a fuel filter clogged condition. This differential pressure applies if a filter is clogged on one or two engines.
If a filter is clogged on one engine only, a L (R) ENGINE FUEL BYPASS advisory message is shown on the EICAS. If the two engines each have their filters clogged at the same time, then a ENGINES FUEL BYPASS caution message is shown on the EICAS.
If an actual filter bypass occurs, a fuel-filter mechanical-differential pressure indicator (DPI) will indicate the event. The DPI is operated on a sensed pressure drop across the fuel filter element to show that the filters capacity has been reached. The indicator will extend to show a blue ring, which gives a visual indication of a filter bypass condition.
The other function of the dual function switch is to measure the temperature of the fuel from the FHOC. The temperature sensor resistive-temperature-device (RTD) sends the fuel temperature at the entry of the fuel filter to the ECU. If the fuel temperature is low, a L (R) ENG FUEL TEMP LOW advisory message is shown on the EICAS. The fuel temperature measurement is used by the aircraft fuel system and also used by the ECU for record functions.
There is also an indication of the fuel filter condition on the FUEL synoptic page in color. The color in the rectangular box shows amber when the fuel is in a bypass condition. The color shows magenta when there is an incorrect signal of fuel bypass condition. The color shows the same as the fuel line upstream in all other conditions.
The EICAS messages that follow are related to the fuel indication system:
| EICAS MESSAGE(S) | LEVEL (COLOR) |
|---|---|
| ENGINES FUEL BYPASS | CAUTION (amber) |
| L FUEL PRESSURE LOW | CAUTION (amber) |
| R FUEL PRESSURE LOW | CAUTION (amber) |
| L ENG FUEL TEMP LOW | ADVISORY (cyan) |
| R ENG FUEL TEMP LOW | ADVISORY (cyan) |
| L ENGINE FUEL BYPASS | ADVISORY (cyan) |
| R ENGINE FUEL BYPASS | ADVISORY (cyan) |
System Interface
The fuel indicating system interfaces with the following systems/components:
- Fuel Feed System
- Engine Indication and Crew Alerting System (EICAS)
- Accessory Gearbox (AGB)
- Fuel Distribution System
- Fuel Control System
- Electronic Control Unit (ECU)